= Solution
Use the projected baseline along the source offset, $b_p$, and $\alpha=10'=2.909\times10^{-3}\,\mathrm{rad}$. The first <bandwidth> null occurs at $\Delta\nu=c/(b_p\alpha)$. A worst-case $b_p=25.46\,\mathrm m$ gives $4.05\,\mathrm{GHz}$. \b[That is a zero of the response, not a good usable <bandwidth>.] A quantitative maximum needs an allowed attenuation.
For example, requiring a response of at least $0.9$ gives $\operatorname{sinc}x=0.9$ at $x\simeq0.787$. Hence
$$
\boxed{\Delta\nu_{10\%}\simeq0.250\frac c{b_p\alpha}\simeq1.01\,\mathrm{GHz}}
$$
for that worst projection. A strictly east-west offset in a sides-aligned square instead samples at most $18\,\mathrm m$, giving a $5.73\,\mathrm{GHz}$ first null and a $1.43\,\mathrm{GHz}$ ten-percent-loss <bandwidth>. The original two-antenna geometry supplies an additional $\cos\theta$ projection; these examples use its maximum magnitude. Thus \b[a <bandwidth> of order $1\,\mathrm{GHz}$] is a reasonable conservative estimate with a stated small-loss criterion, rather than a unique geometry-free limit.
\Image[/past-exam-of-the-frontiers-of-observational-astrophysics-course-of-the-university-of-cambridge/2012/paper-1-array-and-bandwidth.png]
{title=Array baselines and bandwidth decorrelation away from the tracking center}
{description=Original array geometry and rectangular-band sinc calculations for a ten-arcminute offset. The marked ten-percent-loss bandwidth is an illustrative response criterion; the first null is not a usable-bandwidth recommendation.}
To use a much wider total band, channelize the signals before correlation or before irreversible <frequency> averaging. Correlate sufficiently narrow channels, track the reference delay and phase at each <frequency>, and combine frequency-aware visibilities or images. This preserves sensitivity while avoiding <field-dependent averaging losses in radio interferometry>. Merely adding a phase correction after a broad channel has decorrelated cannot recover the lost signal; nor can one tracking delay cancel every sky direction simultaneously.<Field of View by National Radio Astronomy Observatory>[VLA Observational Status Summary 2018A, bandwidth and time-averaging limitations]{ref}
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